Homogeneous charge compression ignition (“HCCI”) is an attractive advanced combustion process that offers potential as a high-efficiency alternative to spark ignition engines. By providing diesel-like efficiencies but with substantially lower NOx and PM emissions, HCCI also offers a low emissions alternative to diesel engines. Unlike conventional diesel combustion, HCCI does not rely on maintaining a flame front. Rather, combustion occurs as the result of spontaneous auto-ignition at multiple points throughout the volume of charge gas. This unique property of HCCI allows the combustion of very lean mixtures or mixtures that are made very dilute by the addition of combustion-product gases (e.g., by exhaust gas recirculation), resulting in low combustion temperatures that dramatically reduce NOx emissions. Also, unlike conventional diesel combustion, the charge is sufficiently well-mixed so that PM emissions are very low. Consequently, HCCI provides a low emissions alternative to conventional diesel engines.
Although the use of conventional diesel fuel or gasoline for HCCI would be desirable since these fuels are readily available, achieving acceptable HCCI operating conditions with these fuels can be difficult. With diesel fuel, elevated temperatures are required before significant vaporization occurs making it difficult to form a premixed near-homogeneous charge. Second, diesel fuel has significant cool-combustion chemistry leading to rapid auto-ignition once compression temperatures exceed about 800° K. This can lead to overly advanced combustion phasing and/or require reduced compression ratios that reduce engine efficiency. Conversely, gasoline can require overly high compression ratios or various techniques to provide significant charge heating.
HCCI combustion is generally characterized as a controlled auto-ignition of a homogenous air/fuel mixture that occurs without a flame front. Relatively high compression ratios, along with high charge dilution, un-throttled operation, and very high rates of combustion provide high efficiency. Resulting low burned-gas temperatures minimize the formation of NOx, and elimination of fuel-rich regions prevents the formation of particulate or soot. There are several versions of HCCI each of which has a distinct acronym, for example: HCCI, PCCI, CAI, PPC, MK, UNIBUS, OKP, and the like. Some diesel engine manufactures are starting to use part-time HCCI (in this case, PCCI) in the diesel marketplace. It is expected that part-time HCCI engines will be used in the gasoline passenger car market in the new future.